As a fluid control valve, a valve such as an exhaust gas recirculation valve is known. The exhaust gas recirculation valve is used in a system in which exhaust gas is circulated and returned to the gas intake side. When this exhaust gas recirculation valve is used to deal with high temperature exhaust gas having a temperature about 800 degrees Celsius, the exhaust gas recirculation valve needs to be configured such that the heat of the exhaust gas is not transmitted to components whose heat-resistant temperature is low and that are included in the actuator unit for driving the valve, as much as possible.
For example, in an exhaust gas recirculation valve disclosed in Patent Literature 1, to protect an actuator unit from heat of exhaust gas having a high temperature a housing that is one of elements constituting a valve main body includes a first housing having an exhaust gas passage and a second housing having a coolant passage.
Moreover, the first housing is made of metal material having higher stiffness or strength at a heated atmosphere than material for the second housing. In addition, the second housing is cooled by coolant fluid for an engine circulating through the coolant passage formed in the second housing. Moreover, a contact area between the first housing and the second housing is reduced as long as possible, and a thermal insulating layer of air is provided between the first housing and the second housing.
Patent Literature 2 discloses an exhaust gas recirculation valve. The exhaust gas recirculation valve has an integrated structure in which an output shaft of an actuator unit is directly gear-engaged with a valve shaft. In this valve, a housing protection component having a cylindrical shape is press-fitted within an exhaust gas passage.
The housing protection component is a component that protects a housing against heat of exhaust gas. Between a part of the housing protection component and a surface of the exhaust gas passage, a thermal insulating layer for reducing heat conduction is provided.
An exhaust gas recirculation valve disclosed in Patent Literature 3 includes a valve-side housing and an actuator-side housing. The valve-side housing has a fluid passage. The actuator-side housing, to which an actuator unit is attached, serves as a gearbox that transmits a rotational driving force to a valve shaft.
The actuator-side housing and the valve-side housing are connected such that a space is provided between them. The space functions as a thermal insulating layer of air. The thermal insulating layer of air suppresses influence of radiant heat from the valve-side housing.
In addition, a coolant passage is formed in the actuator-side housing. The coolant passage reduces heat transfer to a gear mechanism and the actuator unit by its cooling effect.